1. Field of the Invention
The present invention relates to a tool holder which is detachably attached to a spindle of a machine tool, and particularly to a tool holder composed of a main sleeve to be attached to the spindle, and a subholder which is detachably attached to the main sleeve.
2. Description of Related Art
A main spindle of a machine tool such as a machining center is provided at its forward end with a tapered bore for receiving a tool holder. Conventionally, a taper of 7/24, i.e., a taper angle slightly larger than 16.degree. has been widely used as an international standard for the tapered bore. Accordingly, the shank portion of each tool holder is designed to have a taper of 7/24 to conform to the taper of the tapered bore.
A tool holder composed of a main sleeve and a subholder has been used. The main sleeve of the tool holder has a tapered shank which is inserted into a tapered bore of a spindle, and a tapered hole which is provided at the forward end of the tapered shank to receive the subholder. The tapered shank of the main sleeve has the same steep taper as the tapered bore of the spindle (i.e., a taper of 7/24), while the tapered hole of the main sleeve has a gentle taper of 1/10, i.e., a taper angle slightly larger than 5.degree.. The subholder has a hollow tapered shank which has the same taper angle as the tapered hole of the main sleeve. The tapered shank of the subholder is removably inserted into the tapered hole of the main sleeve to fix the subholder to the main sleeve.
The tool holder composed of the main sleeve and the subholder can be attached to a tapered bore which is widely used for spindles of machine tools. Also, since the tapered shank of the subholder is hollow, the tapered shank can be closely fitted into the tapered hole of the main sleeve by utilizing elastic deformation of the tapered shank. This increases the concentricity between the main sleeve and the subholder which are coupled with each other via the gentle taper of 1/10. However, the hollow tapered shank of the subholder tends to elastically deform due to a cutting force in a radial direction so that distortion and bending are produced in the tool holder. Also, it is difficult to obtain a well balanced rotation. Accordingly, the conventional tool holder has the drawback that a workpiece made of metal cannot be machined at high speed using a cutting tool attached to the subholder.